The invention relates to a throttle device for a twin-shafted screw machine having a housing with two mutually penetrating housing bores in which respective screw shafts having mutually meshing screw elements are arranged, each housing bore having a outer gap sleeve substantially tightly abutting the wall of the housing bore, and each screw shaft having an associated inner gap sleeve, the inner and outer gap sleeves being mutually axially displaceable in order to generate or to modify a gap therebetween for throttling the material to be processed, the outer gap sleeves being guided to prevent relative rotation therebetween. Such throttle devices are hereinafter referred to as throttle devices of the type referred to.
In one such throttle device of the type referred to, which is known from German Federal Republic Offenlegungsschrift (Laid Open Patent Application) No. 23 21 325, the surfaces delimiting the gap on the outer gap sleeves and the inner gap sleeves are conical, so that a gap width adjustment is effected by a slight relative axial displacement of said sleeves. The outer gap sleeves are mounted firmly in the relevant housing bore. The gap width adjustment occurs in that, the screw shaft being firmly held, the housing is displaced slightly axially, or the housing being firmly held, the screw shaft is displaced slightly axially. This mode of gap width adjustment produces great disproportionalities between the adjustment stroke on the one hand and the increase in counter-pressure in the material to be treated, which is of course essential to the shearing effect in the gap.
A throttle device for a single-shafted screw machine is known from U.S. Pat. No. 3,780,994, wherein there is mounted on the screw shaft an inner gap sleeve with a circular cylindrical outer circumference, with which there is associated an outer gap sleeve with a circular cylindrical internal bore which is slidable axially in the housing bore by being slid by means of a screw-threaded transmission. For a single-shafted screw machine this throttle device already achieves the advantage that proportionality is obtained between the adjustment stroke and the counterpressure variation in the material to be treated. Such a throttle gap infinitely adjustable in length therefore has a linear throttle characteristic. The disadvantage of this construction lies in the fact that it is unsuitable for multiple-shafted machines, and that the housing bore requires an enlarged cross-section in the region of the throttle device.